Sorry to bring up and old thread, but I think this is gonna be a service to some people. People always ask what to do and some people recommend to put a cover on the tank, but no one actually replies back if they did it or not and what success they had.
Well my house got tented for termites. They used Vikane. I covered the tank with a plastic painters tarp from the local hardware store and taped it around the base of the tank.
Important factors to consider:
1) There was no oxygen line running from the outside as some people have suggested
2) Tarp on tank was NOT super air tight, but i did my best.
3) I did not see my tank for like 17hrs, which I uncovered it afterwards.
4) 72gal reef tank and I only lost one fish.
Please feel free to ask me any questions as I feel that a lot of people would appreciate this information.